1. Field of the Invention
The invention relates to an induction furnace suited to operation at temperatures of around 3000xc2x0 C. and above. It finds particular application in conjunction with the graphitization of pitch fibers and other carbon-containing fibers and will be described with particular reference thereto. It should be appreciated, however, that the furnace is also suited to other high temperature processes, such as halogen purification of graphitic materials to remove metal impurities.
2. Discussion of the Art
Batch induction furnaces have been used for many years for fiber graphitization and other high temperature operations. A typical induction furnace includes an electrically conductive vessel, known as a susceptor. A time-varying electromagnetic field is generated by an alternating current (ac) flowing in an induction heating coil. The magnetic field generated by the coil passes through the susceptor. The magnetic field induces currents in the susceptor, which generate heat. The material to be heated is contained within the susceptor in what is commonly referred to as the xe2x80x9chot zone,xe2x80x9d or hottest part of the furnace.
For operations which require high temperatures, of up to about 3000xc2x0 C., graphite is a preferred material for forming the susceptor, since it is both electrically conductive and able to withstand very high temperatures. There is a tendency, however, for the graphite to sublime, turning to vapor. Sublimation increases markedly as the temperature rises above about 3100xc2x0 C. Because of variations in temperature throughout the susceptor, furnace life at a nominal operating temperature of about 3100xc2x0 C. is typically measured in weeks. Life at 3400xc2x0 C. is often only a matter of hours. Thus, furnaces which are operated at temperatures of over 3000xc2x0 C. tend to suffer considerable downtime for replacement of components.
Graphitization of carbon-containing fibers, in particular, benefits from treatment temperatures of over 3000xc2x0 C. For example, in the formation of lithium batteries, uptake of lithium is dependent on the temperature of graphitization, improving as the graphitization temperature increases. Some improvements in the heat distribution throughout the susceptor have been accomplished by measuring the temperature at different points within the furnace during heating using pyrometers. Different densities of induction power are then delivered to multiple sections of the susceptor along its length, according to the measured temperatures. However, pyrometers are prone to failure and need recalibration over time.
To increase the lifetime of the susceptor, it is desirable to cool the furnace rapidly once the high temperature heating operation is complete. Typically, cooling coils carry water around the furnace. However, because the furnace is generally well insulated, it often takes about a week to cool the furnace down from its operating temperature. In some applications, heat exchangers are employed to speed cooling. In such designs, the furnace is cooled to a temperature of about 1500xc2x0 C. by heat loss through the furnace insulation. Then, valves above and below the hot zone are opened and forced circulation through an external heat exchanger is begun. This system works well for furnaces that are rarely operated above 2800xc2x0 C. In furnaces that are routinely operated above 3000xc2x0 C., the frequent replacement of hot zone components renders these designs expensive to operate. In other designs, the loose insulation material above the furnace is knocked off the furnace to speed cooling. As a result, the insulation needs to be replaced after each furnace run.
The present invention provides a new and improved induction furnace and method of use, which overcome the above-referenced problems, and others.
In accordance with one aspect of the present invention, a furnace is provided. The furnace includes a vessel which defines an interior chamber for receiving items to be treated and a heating means which heats the vessel. A cap selectively closes the vessel interior chamber. A cooling assembly includes a dome which defines a chamber and a lifting mechanism which selectively lifts the cap allowing hot gas to flow from the vessel interior chamber to the dome.
In accordance with another aspect of the present invention, a cooling assembly for a furnace is provided. The cooling assembly includes a dome which defines an interior chamber. A cooling means cools the dome. The assembly includes means for selectively providing fluid communication between a hot zone of the induction furnace and the dome and means for controlling the communicating means in accordance with at least one of a temperature of the hot zone and a temperature of the interior chamber.
In accordance with yet another aspect of the present invention, an induction furnace is provided. The furnace includes a susceptor which defines an interior chamber for receiving items to be treated, the susceptor being formed from graphite. An induction coil induces a current in the susceptor to heat the susceptor. A layer of flexible graphite, exterior to the susceptor, inhibits escape of carbon vapor which has sublimed from the susceptor.
In accordance with yet another aspect of the present invention, a method of operating a furnace is provided. The method includes heating items to be treated in a first chamber which contains a gas and actively cooling a second chamber which contains a gas. The second chamber is selectively fluidly connectable with the first chamber. After the step of heating, the first chamber is cooled by selectively fluidly connecting the first chamber with the second chamber, thereby allowing heat to flow from the gas in the first chamber to the gas in the second chamber.
An advantage of at least one embodiment of the present invention is that significant increases in furnace life are obtained.
Another advantage of at least one embodiment of the present invention is that cool down times are reduced.
Another advantage of at least one embodiment of the present invention is that the cooling assembly is readily removable from the furnace, simplifying removal and replacement of the susceptor and other hot zone components.
Other advantages of at least one embodiment of the present invention derive from greater accuracy in monitoring variations in furnace temperature throughout the furnace.
Still further advantages of the present invention will be readily apparent to those skilled in the art, upon a reading of the following disclosure and a review of the accompanying drawings.